Simultaneous δ15N, δ13C and δ34S measurements of low‐biomass samples using a technically advanced high sensitivity elemental analyzer connected to an isotope ratio mass spectrometer

Conventional simultaneous CNS stable isotope abundance measurements of solid samples usually require high sample amounts, up to 1 mg carbon, to achieve exact analytical results. This rarely used application is often impaired by high C:S element ratios when organic samples are analyzed and problems such as incomplete conversion into sulphur dioxide occur during analysis. We introduce, as a technical innovation, a high sensitivity elemental analyzer coupled to a conventional isotope ratio mass spectrometer, with which CNS‐stable isotope ratios can be determined simultaneously in samples with low carbon content (<40 µg C corresponding to ～100 µg dry weight). The system includes downsized reactors, a temperature program‐controlled gas chromatography (GC) column and a cryogenic trap to collect small amounts of sulphur dioxide. This modified application allows for highly sensitive measurements in a fully automated operation with standard deviations better than ±0.47‰ for δ¹⁵N and δ³⁴S and ±0.12‰ for δ¹³C (n = 127). Samples collected from one sampling site in a Baltic fjord within a short time period were measured with the new system to get a first impression of triple stable isotope signatures. The results confirm the potential of using δ³⁴S as a stable isotope tracer in combination with δ¹⁵N and δ¹³C measurements to improve discrimination of food sources in aquatic food webs. Copyright © 2009 John Wiley & Sons, Ltd.     

On the calibration of continuous,high‐precision δ18O and δ2H measurements using an off‐axis integrated cavity output spectrometer

The ¹⁸O and ²H of water vapor serve as powerful tracers of hydrological processes. The typical method for determining water vapor δ¹⁸O and δ²H involves cryogenic trapping and isotope ratio mass spectrometry. Even with recent technical advances, these methods cannot resolve vapor composition at high temporal resolutions. In recent years, a few groups have developed continuous laser absorption spectroscopy (LAS) approaches for measuring δ¹⁸O and δ²H which achieve accuracy levels similar to those of lab‐based mass spectrometry methods. Unfortunately, most LAS systems need cryogenic cooling and constant calibration to a reference gas, and have substantial power requirements, making them unsuitable for long‐term field deployment at remote field sites. A new method called Off‐Axis Integrated Cavity Output Spectroscopy (OA‐ICOS) has been developed which requires extremely low‐energy consumption and neither reference gas nor cryogenic cooling. In this report, we develop a relatively simple pumping system coupled to a dew point generator to calibrate an ICOS‐based instrument (Los Gatos Research Water Vapor Isotope Analyzer (WVIA) DLT‐100) under various pressures using liquid water with known isotopic signatures. Results show that the WVIA can be successfully calibrated using this customized system for different pressure settings, which ensure that this instrument can be combined with other gas‐sampling systems. The precisions of this instrument and the associated calibration method can reach ～0.08‰ for δ¹⁸O and ～0.4‰ for δ²H. Compared with conventional mass spectrometry and other LAS‐based methods, the OA‐ICOS technique provides a promising alternative tool for continuous water vapor isotopic measurements in field deployments. Copyright © 2009 John Wiley & Sons, Ltd.     

Minimization of sample requirement for δ18O in benzoic acid

The measurement of the oxygen stable isotope content in organic compounds has applications in many fields, ranging from paleoclimate reconstruction to forensics. Conventional High‐Temperature Conversion (HTC) techniques require >20 µg of O for a single δ¹⁸O measurement. Here we describe a system that converts the CO produced by HTC into CO₂ via reduction within a Ni‐furnace. This CO₂ is then concentrated cryogenically, and 'focused' into the isotope ratio mass spectrometry (IRMS) source using a low‐flow He carrier gas (6–8 mL/min). We report analyses of benzoic acid (C₇H₆O₂) reference materials that yielded precise δ¹⁸O measurement down to 1.3 µg of O, suggesting that our system could be used to decrease sample requirement for δ¹⁸O by more than an order of magnitude. Copyright © 2010 John Wiley & Sons, Ltd.     

Simultaneous determination of δ15N and δ18O of N2O and δ13C of CH4 in nanomolar quantities from a single water sample

We have developed a rapid, sensitive, and automated analytical system to simultaneously determine the concentrations and stable isotopic compositions (δ¹⁵N, δ¹⁸O, and δ¹³C) of nanomolar quantities of nitrous oxide (N₂O) and methane (CH₄) in water, by combining continuous‐flow isotope‐ratio mass spectrometry and a helium‐sparging system to extract and purify the dissolved gases. Our system, which is composed of cold traps and a capillary gas chromatograph that use ultra‐pure helium as the carrier gas, achieves complete extraction of N₂O and CH₄ in a water sample and separation among N₂O, CH₄, and the other component gases. The flow path following exit from the gas chromatograph was periodically changed to pass the gases through the combustion furnace to convert CH₄ and the other hydrocarbons into CO₂, or to bypass the combustion furnace for the direct introduction of eluted N₂O into the mass spectrometer, for determining the stable isotopic compositions through monitoring the ions of m/z 44, 45, and 46 of CO and N₂O⁺. The analytical system can be operated automatically with sequential software programmed on a personal computer. Analytical precisions better than 0.2‰ and 0.3‰ and better than 1.4‰ and 2.6‰ were obtained for the δ¹⁵N and δ¹⁸O of N₂O, respectively, when more than 6.7 nmol and 0.2 nmol of N₂O, respectively, were injected. Simultaneously, analytical precisions better than 0.07‰ and 2.1‰ were obtained for the δ¹³C of CH₄ when more than 5.5 nmol and 0.02 nmol of CH₄, respectively, were injected. In this manner, we can simultaneously determine stable isotopic compositions of a 120 mL water sample with concentrations as low as 1.7 nmol/kg for N₂O and 0.2 nmol/kg for CH₄. Copyright © 2010 John Wiley & Sons, Ltd.     

A dynamic soil chamber system coupled with a tunable diode laser for online measurements of δ13C, δ18O,and efflux rate of soil‐respired CO2

High frequency observations of the stable isotopic composition of CO₂ effluxes from soil have been sparse due in part to measurement challenges. We have developed an open‐system method that utilizes a flow‐through chamber coupled to a tunable diode laser (TDL) to quantify the rate of soil CO₂ efflux and its δ¹³C and δ¹⁸O values (δ¹³C_R and δ¹⁸O_R, respectively). We tested the method first in the laboratory using an artificial soil test column and then in a semi‐arid woodland. We found that the CO₂ efflux rates of 1.2 to 7.3 µmol m^?2 s^?1 measured by the chamber‐TDL system were similar to measurements made using the chamber and an infrared gas analyzer (IRGA) (R² = 0.99) and compared well with efflux rates generated from the soil test column (R² = 0.94). Measured δ¹³C and δ¹⁸O values of CO₂ efflux using the chamber‐TDL system at 2 min intervals were not significantly different from source air values across all efflux rates after accounting for diffusive enrichment. Field measurements during drought demonstrated a strong dependency of CO₂ efflux and isotopic composition on soil water content. Addition of water to the soil beneath the chamber resulted in average changes of +6.9 µmol m^?2 s^?1, ?5.0‰, and ?55.0‰ for soil CO₂ efflux, δ¹³C_R and δ¹⁸O_R, respectively. All three variables initiated responses within 2 min of water addition, with peak responses observed within 10 min for isotopes and 20 min for efflux. The observed δ¹⁸O_R was more enriched than predicted from temperature‐dependent H₂O‐CO₂ equilibration theory, similar to other recent observations of δ¹⁸O_R from dry soils (Wingate L, Seibt U, Maseyk K, Ogee J, Almeida P, Yakir D, Pereira JS, Mencuccini M. Global Change Biol. 2008; 14: 2178). The soil chamber coupled with the TDL was found to be an effective method for capturing soil CO₂ efflux and its stable isotope composition at high temporal frequency. Published in 2010 by John Wiley & Sons, Ltd.     

Determination of δ18O in soils: measuring conditions and a potential application

The stable oxygen isotope signature (δ¹⁸O) of soil is expected to be the result of a mixture of components within the soil with varying δ¹⁸O signatures. Thus, the δ¹⁸O of soils should provide information about the soil's substrate, especially about the relative contribution of organic matter versus minerals. As there is no standard method available for measuring soil δ¹⁸O, the method for the measurement of single components using a high‐temperature conversion elemental analyser (TC/EA) was adapted. We measured δ¹⁸O in standard materials (IAEA 601, IAEA 602, Merck cellulose) and soils (organic and mineral soils) in order to determine a suitable pyrolysis temperature for soil analysis. We consider a pyrolysis temperature suitable when the yield of signal intensity (intensity of mass 28 per 100 µg) is at a maximum and the acquired raw δ¹⁸O signature is constant for the standard materials used and when the quartz signal from the soil is still negligible. After testing several substances within the temperature range of 1075 to 1375°C we decided to use a pyrolysis temperature of 1325°C for further measurements. For the Urseren Valley we have found a sequence of increasing δ¹⁸O signatures from phyllosilicates to upland soils, wetland soils and vegetation. Our measurements show that the δ¹⁸O values of upland soil samples differ significantly from wetland soil samples. The latter can be related to the changing mixing ratio of the mineral and organic constituents of the soil. For wetlands affected by soil erosion, we have found intermediate δ¹⁸O signatures which lie between typical signatures for upland and wetland sites and give evidence for the input of upland soil material through erosion. Copyright © 2008 John Wiley & Sons, Ltd.     

Comprehensive inter‐laboratory calibration of reference materials for δ18O versus VSMOW using various on‐line high‐temperature conversion techniques

Internationally distributed organic and inorganic oxygen isotopic reference materials have been calibrated by six laboratories carrying out more than 5300 measurements using a variety of high‐temperature conversion techniques (HTC) a in an evaluation sponsored by the International Union of Pure and Applied Chemistry (IUPAC). To aid in the calibration of these reference materials, which span more than 125‰, an artificially enriched reference water (δ¹⁸O of +78.91‰) and two barium sulfates (one depleted and one enriched in ¹⁸O) were prepared and calibrated relative to VSMOW2 b and SLAP reference waters. These materials were used to calibrate the other isotopic reference materials in this study, which yielded:

Hydrogen and oxygen isotopes of water from inclusions in minerals: design of a new crushing system and on‐line continuous‐flow isotope ratio mass spectrometric analysis

An analytical line for stable isotope analyses of water recovered from fluid inclusions in minerals was built and successfully tested. The line is based on the principle of continuous‐flow analysis of water via high‐temperature reduction on glassy carbon. It includes a custom‐designed set of high‐efficiency crushers and a cryo‐focusing cell. This paper provides details of the line design and discusses strategies for line conditioning and mitigation of memory effects. The line allows measurements of hydrogen and oxygen isotopes during a single acquisition. The precision of the analyses depends on the amount of water released from the inclusions. The best results are obtained for samples containing at least 0.1–0.2 µL (0.06–0.11 µmol) H₂O. For such samples precision is better than 1.5‰ for δD and 0.5‰ for δ¹⁸O (1σ). Smaller amounts of water can be measured but at lower precision. Analyses of modern calcite formed under stable conditions in a deep cave allowed assessment of the accuracy of the analyses. The δD values measured in fluid inclusions of this working standard match the δD value of the parent water, and the oxygen isotope values agree within ca. 0.5‰. This indicates that fluid inclusions trapped in calcite at near‐ambient temperatures (e.g. speleothems and low‐temperatures phreatic calcite) faithfully preserve the original isotopic composition of the parent waters. Copyright © 2009 John Wiley & Sons, Ltd.     

The hydrological response of heavy clay grassland soils to rainfall in south‐west England using δ2H

Stable isotopes of water have been previously used in catchment studies to separate rain‐event water from pre‐event groundwater. However, there are a lack of studies at the smaller scale looking at the separation of event water from pre‐event water. This is particularly relevant for heavy clay soil systems through which the movement of water is uncertain but is thought to be rainwater‐dominated. The data presented here were collected at a rural site in the south‐west of England. The historic rainfall at the site was isotopically varied but similar to the global meteoric water line, with annual weighted means of ?37‰ for δ²H and ?5.7‰ for δ¹⁸O and with no seasonal variation. Drainage was sampled from the inter‐flow (surface runoff + lateral through‐flow) and drain‐flow (55 cm deep mole drains) pathways of two 1 ha lysimeters during two rainfall events, which had δ²H values of ?68‰ and ?92‰, respectively. The δ²H values of the lysimeter drainage water suggest that there was no contribution of event water during the first, small discharge (Q) event; however, the second larger event did show isotopic variation in δ²H values negatively related to Q indicating that rainwater was contributing to Q. A hydrograph separation indicated that only 49–58% of the inter‐flow and 18–25% of the drain‐flow consisted of event water. This was surprising given that these soil types are considered retentive of soil water. More work is needed on heavy clay soils to understand better the nature of water movement from these systems. Copyright © 2010 John Wiley & Sons, Ltd.     

An online method combining a Gasbench II with continuous flow isotope ratio mass spectrometry to determine the content and isotopic compositions of minor amounts of carbonate in silicate rocks

An online method using continuous flow isotope ratio mass spectrometry (CF‐IRMS) interfaced with a Gasbench II device was established to analyze carbon and oxygen isotopic compositions and to estimate the content of minor amounts of carbonate in silicate rocks. The mixtures of standard materials and high‐purity quartz are firstly used to calibrate different quantities of carbonate in silicates. The results suggest that the accuracy and precision of the online analysis are both better than those obtained using an offline method. There is a positive correlation between the carbonate weight and the Mass44 ion beam intensity (or peak area). When the weight of carbonate in the mixtures is greater than 70 µg (equal to ～1800 mV Mass44 ion beam intensity), the δ¹³C and δ¹⁸O values of samples usually have accuracy and precision of ±0.1‰ and ±0.2‰ (1σ), respectively. If the weight is less than 70 µg, some limitations (e.g., not perfectly linear) are encountered that significantly reduce the accuracy and precision. The measured δ¹⁸O values are systematically lower than the true values by ?0.3 to ?0.7‰; the lower the carbonate content, the lower the measured δ¹⁸O value. For samples with lower carbonate content, the required phosphoric acid doses are higher and more oxygen isotope exchanges with the water in the phosphoric acid. To guarantee accurate results with high precision, multiple analyses of in‐house standards and an artificial MERCK sample with δ¹³C values from ?35.58 to 1.61‰ and δ¹⁸O from 6.04 to 18.96‰ were analyzed simultaneously with the unknown sample. This enables correction of the measured raw data for the natural sample based on multiple‐point normalization. The results indicate that the method can be successfully applied to a range of natural rocks. Copyright © 2010 John Wiley & Sons, Ltd.     

Consistent predictable patterns in the hydrogen and oxygen stable isotope ratios of animal proteins consumed by modern humans in the USA

Published datasets of proteinaceous animal tissues suggest that co‐variation between amino acid hydrogen (δ²H) and oxygen (δ¹⁸O) isotope ratios is a common feature in systems where isotopic variation is driven by geographic or temporal variation in the δ²H and δ¹⁸O values of environmental water. This has led to the development of models relating tissue δ²H and δ¹⁸O values to those of water, with potential application in a number of fields. However, the strength and ubiquity of the influence of environmental water on protein isotope ratios across taxonomic groups, and thus the relevance of predictive models, is an open question. Here we report strong co‐variation of δ²H and δ¹⁸O values across a suite of terrestrial and aquatic animal meats purchased in American food markets, including beef, poultry (chicken and turkey), chicken eggs, pork, lamb, freshwater fish, and marine fish. Significant isotope co‐variation was not found for small collections of marine bivalves and crustaceans. These results imply that isotopic signals from environmental water were propagated similarly through most of the diverse natural and human‐managed foodwebs represented by our samples. Freshwater fish had the largest variation in δ²H and δ¹⁸O values, with ranges of 121 ‰ and 19.2 ‰, respectively, reflecting the large isotopic variation in environmental freshwaters. In contrast marine animals had the smallest variation for both δ²H (7 ‰ range, crustaceans) and δ¹⁸O (3.0 ‰ range, bivalves) values. Known‐origin beef samples demonstrated direct relationships between the variance of environmental water isotope ratios and that of collected meats. Copyright © 2011 John Wiley & Sons, Ltd.     

δ13C, δ15N and δ2H isotope ratio mass spectrometry of ephedrine and pseudoephedrine: application to methylamphetamine profiling

Conventional chemical profiling of methylamphetamine has been used for many years to determine the synthetic route employed and where possible to identify the precursor chemicals used. In this study stable isotope ratio analysis was investigated as a means of determining the origin of the methylamphetamine precursors, ephedrine and pseudoephedrine. Ephedrine and pseudoephedrine may be prepared industrially by several routes. Results are presented for the stable isotope ratios of carbon (δ¹³C), nitrogen (δ¹⁵N) and hydrogen (δ²H) measured in methylamphetamine samples synthesized from ephedrine and pseudoephedrine of known provenance. It is clear from the results that measurement of the δ¹³C, δ¹⁵N and δ²H stable isotope ratios by elemental analyzer/thermal conversion isotope ratio mass spectrometry (EA/TC‐IRMS) in high‐purity methylamphetamine samples will allow determination of the synthetic source of the ephedrine or pseudoephedrine precursor as being either of a natural, semi‐synthetic, or fully synthetic origin. Copyright © 2009 Commonwealth of Australia. Published by John Wiley & Sons, Ltd.     

A nuclear magnetic resonance (1H and 13C) and isotope ratio mass spectrometry (δ13C, δ2H and δ18O) study of Andalusian olive oils

We have determined δ¹³C, δ²H and δ¹⁸O isotopic abundances in Andalusian olive oils. In addition, the fatty acid composition and the distribution of isomers at positions 1,3 and 2 of glycerol were determined by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, respectively. Isotopic results obtained for four series of oil samples extracted from olives harvested in the 2004/05 and 2005/06 seasons are discussed in terms of olive variety, ripeness, geographical origin, fatty acid composition and growing altitude. A distinction was also established between olives grown in irrigated and in dry land by studying selected samples of the previous series and others from the 2005/06, 2006/07, 2007/08 and 2008/09 seasons. The results showed that olive ripeness does not influence the abundance of any of the three isotopes studied. On the other hand, the olive variety influences the abundance of the oxygen and hydrogen isotopes, and also, less markedly, that of carbon. No clear‐cut effect of height or latitude on isotope values is observed, probably because the olive variety also changes with height and latitude, thus masking such influences. The oil samples from dryland‐grown olives had increased δ¹³C values relative to irrigation‐grown olives. In addition, no definite relationship appears to exist between isotope distribution and fatty acid composition. Finally, oil samples from olives harvested in the 2005/06 season in Italy could be distinguished from those from Spain in terms of their isotopic values (δ²H mainly). Copyright © 2010 John Wiley & Sons, Ltd.     

Calcite‐CO2 mixed into CO2‐free air: a new CO2‐in‐air stable isotope reference material for the VPDB scale

In order to generate a reliable and long‐lasting stable isotope ratio standard for CO₂ in samples of clean air, CO₂ is liberated from well‐characterized carbonate material and mixed with CO₂‐free air. For this purpose a dedicated acid reaction and air mixing system (ARAMIS) was designed. In the system, CO₂ is generated by a conventional acid digestion of powdered carbonate. Evolved CO₂ gas is mixed and equilibrated with a prefabricated gas comprised of N₂, O₂, Ar, and N₂O at close to ambient air concentrations. Distribution into glass flasks is made stepwise in a highly controlled fashion. The isotopic composition, established on automated extraction/measurement systems, varied within very small margins of error appropriate for high‐precision air‐CO₂ work (about ±0.015‰ for δ¹³C and ±0.025‰ for δ¹⁸O). To establish a valid δ¹⁸O relation to the VPDB scale, the temperature dependence of the reaction between 25 and 47°C has been determined with a high level of precision. Using identical procedures, CO₂‐in‐air mixtures were generated from a selection of reference materials; (1) the material defining the VPDB isotope scale (NBS 19, δ¹³C = +1.95‰ and δ¹⁸O = ?2.2‰ exactly); (2) a local calcite similar in isotopic composition to NBS 19 (‘MAR‐J1’, δ¹³C = +1.97‰ and δ¹⁸O = ?2.02‰), and (3) a natural calcite with isotopic compositions closer to atmospheric values (‘OMC‐J1’, δ¹³C = ?4.24‰ and δ¹⁸O = ?8.71‰). To quantitatively control the extent of isotope‐scale contraction in the system during mass spectrometric measurement other available international and local carbonate reference materials (L‐SVEC, IAEA‐CO‐1, IAEA‐CO‐8, CAL‐1 and CAL‐2) were also processed. As a further control pure CO₂ reference gases (Narcis I and II, NIST‐RM 8563, GS19 and GS20) were mixed with CO₂‐free synthetic air. Independently, the pure CO₂ gases were measured on the dual inlet systems of the same mass spectrometers. The isotopic record of a large number of independent batches prepared over the course of several months is presented. In addition, the relationship with other implementations of the VPDB‐scale for CO₂‐in‐air (e.g. CG‐99, based on calibration of pure CO₂ gas) has been carefully established. The systematic high‐precision comparison of secondary carbonate and CO₂ reference materials covering a wide range in isotopic composition revealed that assigned δ‐values may be (slightly) in error. Measurements in this work deviate systematically from assigned values, roughly scaling with isotopic distance from NBS 19. This finding indicates that a scale contraction effect could have biased the consensus results. The observation also underlines the importance of cross‐contamination errors for high‐precision isotope ratio measurements. As a result of the experiments, a new standard reference material (SRM), which consists of two 5‐L glass flasks containing air at 1.6 bar and the CO₂ evolved from two different carbonate materials, is available for distribution. These ‘J‐RAS’ SRM flasks (‘Jena‐Reference Air Set’) are designed to serve as a high‐precision link to VPDB for improving inter‐laboratory comparability. a Copyright © 2005 John Wiley & Sons, Ltd.     

Revised δ34S reference values for IAEA sulfur isotope reference materials S‐2 and S‐3

Revised δ³⁴S reference values with associated expanded uncertainties (95% confidence interval (C.I.)) are presented for the sulfur isotope reference materials IAEA‐S‐2 (22.62 ± 0.16‰) and IAEA‐S‐3 (−32.49 ± 0.16‰). These revised values are determined using two relative‐difference measurement techniques, gas source isotope ratio mass spectrometry (GIRMS) and double‐spike multi‐collector thermal ionization mass spectrometry (MC‐TIMS). Gas analyses have traditionally been considered the most robust for relative isotopic difference measurements of sulfur. The double‐spike MC‐TIMS technique provides an independent method for value‐assignment validation and produces revised values that are both unbiased and more precise than previous value assignments. Unbiased δ³⁴S values are required to anchor the positive and negative end members of the sulfur delta (δ) scale because they are the basis for reporting both δ³⁴S values and the derived mass‐independent Δ³³S and Δ³⁶S values. Published in 2009 by John Wiley & Sons, Ltd.     

Calibration of δ17O and δ18O of international measurement standards – VSMOW,VSMOW2, SLAP,and SLAP2

Due to exhaustion of the two primary calibration materials, Vienna Standard Mean Ocean Water (VSMOW) and Standard Light Antarctic Precipitation (SLAP), two replacement materials, VSMOW2 and SLAP2, were created with isotopic compositions as close as possible to the original standards in their D/H and ¹⁸O/¹⁶O ratios. Measurements of the δ¹⁷O composition constitute therefore an appropriate independent check of the achieved isotopic adjustment. Aliquots from ampoules of VSMOW, VSMOW2, SLAP, and SLAP2 were fluorinated by BrF₅ and analyzed using a dual‐inlet Delta E mass spectrometer. VSMOW2 and SLAP2 were found to be indistinguishable from VSMOW and SLAP, respectively, in their δ¹⁷O and δ¹⁸O values within measurement uncertainties. This result is a confirmation of the successful isotopic matching of VSMOW2 and SLAP2 to their predecessors. Further checks of the δ¹⁷O value of SLAP2 seem desirable. Copyright © 2010 John Wiley & Sons, Ltd.     

H2O‐δD‐FeIII relations of dehydrogenation and dehydration processes in magmatic amphiboles

Stable isotope compositions of a suite of magmatic amphiboles from alkaline basalts and andesitic rocks were examined to constrain the effects of degassing processes on the hydrogen isotope compositions. The Fe³⁺ (as Fe³⁺/Fe_total) and H₂O contents, as well as the H isotope compositions of the amphiboles, differ markedly (27–58%, 0.5–2.2 wt%, ?107 to ?15‰, respectively) but indicate systematic variations. The observed trends can be explained either as dehydrogenation or dehydration processes, both of which are coupled to oxidation processes, the latter most probably related to O^2? substitution within amphiboles. The dehydrogenation‐dehydration models can be used to assess the primary compositions of the magmas. As an important example, δD values of amphiboles of Martian meteorites are discussed in a similar context. Copyright © 2006 John Wiley & Sons, Ltd.     

The Positional δ(18O) Values of Extracted and Synthetic Vanillin

The positional δ(¹⁸O) values of vanillin ( 1 ) of different origins have been determined from the global values of 2‐methoxy‐4‐methylphenol ( 4 ), obtained from 1 upon Clemensen reduction, and of 3‐methylanisole ( 5 ), obtained from 4 by removal of the phenolic O‐atom. By these means, it is possible to differentiate samples of 1 of synthetic origin from those extracted from Vanilla plants or produced from lignin by chemical oxidation. The main difference between the samples derived from guaiacol and those possessing the aromatic moiety of natural origin is in the enrichment values of the O‐atoms at C(3) and C(4), while the extractive materials from the pods are distinguished from the product from lignin on the basis of the carbonyl oxygen δ(¹⁸O) values, ranging from +25.5 and +26.2 in the natural material to +19.7‰ in the lignan‐based sample. The values for the phenolic O‐atom vary from +8.9 and +12‰ of the synthetic materials to +6.5, +5.3, and +6.3‰, respectively, of the sample from lignin and the two samples from Vanilla pods,whereas the MeO O‐atoms show the following values for the same compounds: −2.9, −3.2, +3.5, +3.1, and +2.3‰, respectively. This study indicates the significance of the positional δ(¹⁸O) values of polyoxygenated compounds for the definition of their origin.     

Continuous analysis of δ18O and δD values of water by diffusion sampling cavity ring‐down spectrometry: a novel sampling device for unattended field monitoring of precipitation,ground and surface waters

A novel sampling device suitable for continuous, unattended field monitoring of rapid isotopic changes in environmental waters is described. The device utilises diffusion through porous PTFE tubing to deliver water vapour continuously from a liquid water source for analysis of δ¹⁸O and δD values by Cavity Ring‐Down Spectrometry (CRDS). Separation of the analysed water vapour from non‐volatile dissolved and particulate contaminants in the liquid sample minimises spectral interferences associated with CRDS analyses of many aqueous samples. Comparison of isotopic data for a range of water samples analysed by Diffusion Sampling‐CRDS (DS‐CRDS) and Isotope Ratio Mass Spectrometry (IRMS) shows significant linear correlations between the two methods allowing for accurate standardisation of DS‐CRDS data. The internal precision for an integration period of 3 min (standard deviation (SD) = 0.1 ‰ and 0.3 ‰ for δ¹⁸O and δD values, respectively) is similar to analysis of water by CRDS using an autosampler to inject and evaporate discrete water samples. The isotopic effects of variable air temperature, water vapour concentration, water pumping rate and dissolved organic content were found to be either negligible or correctable by analysis of water standards. The DS‐CRDS system was used to analyse the O and H isotope composition in short‐lived rain events. Other applications where finely time resolved water isotope data may be of benefit include recharge/discharge in groundwater/river systems and infiltration‐related changes in cave drip water. Copyright © 2011 John Wiley & Sons, Ltd.     

The effects of differentiated water supply after anthesis and nitrogen fertilization on δ15N of wheat grain

The δ¹⁵N signature of plants integrates various processes in soil and plant. In this study, the effect of different water regimes applied during the period of grain growth of winter wheat on grain δ¹⁵N was examined in a 4‐year field experiment. The treatments comprised water shortage (S), an ample water supply (W), and rain‐fed crop (R). Zero fertilization (N0) and 200 kg N.ha^?1 in mineral fertilizer (N1) treatments were studied. The grain ¹⁵N was determined during grain growth and at maturity. The water regime, nitrogen application and year had a significant effect on mature grain δ¹⁵N (p < 0.001). Water and nitrogen explained 54.6% of the variability of δ¹⁵N in the experiment, the year accounted for 10.7% and the interactions for another 19.6% of the total variability. The analysis of non‐mature grain δ¹⁵N showed significant effects of N and year but not of water. Nitrogen fertilization reduced the δ¹⁵N of mature grain in years by 0.7–6.3‰ in comparison with N0 plants; the reduction was more pronounced under stress (average reduction by 4.1‰) than under rain‐fed (2.4‰) and ample water supply (2.2‰). Water stress decreased the grain δ¹⁵N in fertilized wheat, by 0.1–2.1‰ and 0.6–3.6‰ in experimental years, on average by 1.30‰ and 1.79‰ in comparison with the R and W water supply, respectively. The effect of water supply was not significant in non‐fertilized wheat. A significant negative linear relationship between grain N concentration and δ¹⁵N in maturity or during the grain growth (R² = 0.83, R² = 0.76, respectively) was found. The observed sources of grain δ¹⁵N variability should be taken into consideration when analyzing and interpreting the data on the δ¹⁵N signature of plant material from field conditions. Copyright © 2010 John Wiley & Sons, Ltd.